High volume production paint operations require the changing of colors for successively painted products. For example, in an automotive production plant, car bodies are scheduled for production in an order influenced by many factors, where color is only one of such factors. As the bodies enter the paint booth, the paint equipment are set by their controllers to paint the required colors. Colors may be changed as often as for each successive car body entering the paint booth, or, the car bodies may be sequenced such that as many successive cars as possible are painted with the same color.
When colors are changed, the paint line extending from the paint color changer to the spray nozzle is thoroughly cleaned with paint solvent before the new color is introduced to fill the line to the nozzle. Since the paint in the lines cannot be fully utilized, much paint is wasted with every color change. This is a costly and environmentally sensitive process and is highly critical that its frequency and waste be minimized.
Usually the sequence of changing colors is as follows:
1. When color change is due, the current color valve is turned OFF at an interval, Ts, before shutting off the spray. Ts is calculated to allow most of the paint remaining in the delivery line to be use productively before the line is cleaned. PA0 2. A low pressure air, (i.e. "soft air"), valve is turned on to push the current color paint with air, having a pressure equal to the paint pressure, and use the paint in the line for productive painting. After an interval, Ts, the object of the painting is fully painted and the spray nozzle, (i.e. spray gun), is turned OFF. Some paint remains in the line and the internal line walls are usually covered with a layer of unused paint. PA0 3. The paint delivery line is then cleaned by turning the purge cycle on. This opens an excess paint return line to a dump tank, inject solvent, or successive bursts of solvent and high pressure air, into the line and the spray nozzle, and dumps the solvent and air into the dump tank until the line is clear. PA0 4. The line is filled with the new color paint, pushing out any excess solvent through the nozzle or to the dump tank.
Variations of this cycle exist to minimize the time required to empty and clean the line, or to minimize the wasted paint and solvent used in the process.
A typical prior art color changer is represented diagrammatically in FIG. 1.
A series of pneumatically operated two-position valves 10, 12, 14 and 16 communicate solvent source lines and paint source lines 18, 20, 22 and 24 respectively, to a common manifold, generally indicated at 26. The paint valves include return circulation lines 28, 30 and 32 which allow the paint to flow continuously back to central storage tanks regardless of paint utilization. This prevents pain pigments from settling and causing uneven paint coloring. Air lines 34 and 36 also communicate low and high pressure air, respectively, to the terminal end of the common manifold.
The manifold 26 is also communicated through the tubing of a paint delivery line 38 to a spray gun/nozzle 40. The spray gun 40 may also be fitted with a dump valve 42 and a return line 44 that allow excess paint and cleaning fluids to be returned to a dump tank 46.
Control signals, such as on lines 48, 50 and 52, allow a central programmable controller 54 to actuate any of the paint, solvent, air or spray gun actuation valves to effect the desired paint flow, paint changing, and line cleaning operations.
The method of prior art color change is represented in FIG. 2, with a flow chart. Its sequence is described by explaining the blocks of the flow chart.
At block 56, the sequence starts with a clear line. At the beginning of a shift, this is the prevailing condition as the line would have been cleaned at the end of the prior shift.
At block 58, the desired paint color valve is turned ON to pass the desired color paint in the paint delivery line 38. The dump valve 42 and/or the gun nozzle 40 is turned ON to relieve the pressure ahead of the flow of paint. The paint flows from one of the valves 12, 14 through 16 of FIG. 1 through the manifold 26 through the paint delivery line 38 to the spray gun 40 or the dump valve 42 for excess recovery. When an estimated amount of paint has flown through the desired valve, estimated by metered timing of the flow, the spray gun 40 and the dump valve 42 are turned OFF and the system is ready for productive painting with the desired color.
At block 60, with the line 38 filled, the dump valve 42 is turned OFF and the object may be sprayed by turning the gun 40 ON and OFF as necessary.
At block 62, color change is usually commanded by the programmable controller 54 of FIG. 1 ahead of the desired timing of productive painting with the current paint color. This allows the paint filling the delivery line 38 to be utilized. If color change is not due, the current paint continues to be used.
At block 64, when color change is due, the paint delivery line is cleared of the current paint. The current valve is then turned OFF to stop the flow of the current paint, and, simultaneously, a low pressure air valve 65 is turned ON for soft air push-out. Air pressure is usually the same as the paint line pressure, hence allowing continuity in the flow rate of the paint through the nozzle 40.
At block 66, when the paint object is fully painted, the flow of soft air is stopped. This is usually timed to leave some paint in the line 38 which is not utilized.
At block 68, the excess paint is recovered by turning the dump valve 42 ON and a high pressure air valve 69 ON.
At block 70, line 38 is then cleaned by sequencing the alternate opening and closing of hard air and solvent valves 69 and 70, respectively, while the dump valve 42 is ON. This sequence sends slugs of solvent through the line 38 at high speed which is effective in cleaning. After a predetermined period, determined by experiment, the line 38 is assumed clean and ready for filling by the new color.
At block 72, with the line 38 clean, the nozzle 40 is cleared by turning the gun 40 ON, and passing high pressure air, carrying solvent from the line 38 through the nozzle 40.
At block 74, if it is the end of the shift, the system is stopped with the line 38 and nozzle 40 clear and ready for the next shift, otherwise the cycle is repeated and the line 38 is filled with the new color.
Variations of this method exist. For example, U.S Pat. No. 4,902,352 discloses a paint color change system in which the paint flow passage of the paint supply line leading to a paint atomizer is cleaned using a scrubbing medium comprising a high-pressure air containing an atomized solvent. An attempt to parallel two delivery lines so one can be cleaned while the other is being utilized for productive painting is disclosed in U.S. Pat. No. 4,487,367.
U.S. Pat. Nos. 3,108,012 to Curtis; 3,432,383 to Russell; 3,562,014 to Childers et al; 4,418,747 to Baron et al; and 4,898,197 to Barry et al disclose a variety of differently configured slugs used for cleaning various types of rigid hollow tubing, pipeline and the like.
U.S. Pat. Nos. 4,508,266 to Saito et al; 4,657,047 to Kolibas; 4,700,896 to Takeuchi et al; 4,846,226 to Merritt; and 4,909,180 to Oishi et al disclose various types of color changers for paint systems generally related to the present invention.
The prior art color change methods share one or more serious shortcomings. For example, it has been shown that for the commonly used high-solids paint, almost 40% of the paint in the line adheres to the walls of the tubing and is not used for productive painting. In some applications having long paint lines, this represents a large percentage (i.e. 20%-40%) of the paint consumed in production. Much is wasted in addition to the loss of this paint. The cost of cleaning the paint with solvents, recovering the paint and the cleaning solvents for environmental protection, and in finding means for environmentally safe waste dumping for recovery is high.
Also, the time needed for cleaning the line and refilling it is a time lost of the productive time of the paint equipment and facilities. Since in a typical automotive assembly plant this time could amount to 15 seconds of each minute available for production, the paint equipment could lose as much as 25% of its productive capacity. This includes the large investment made in human resources, paint booths, robots, paint circulation and spray equipment, ovens, conveyors, etc.
Finally, the equipment used to improve the performance of color changers involves the use of a multiplicity of valves and control devices which are complex, costly, and prone to failure. Paint leaks and control failures are common occurrences in such systems.